Manual gestures are readily available for communicative purposes even when individuals have had no prior experience with conventional sign language. Gestures are particularly well-suited for communication because of their iconicity or transparency of form. Previous research indicates that the gestures created by novice signers, unlike the gestures by native signers, are neither stable in form nor integrated into a coherent language system. The present study explores the role an active partner plays in developing an internally consistent and grammatically complex system of language by introducing a naive recipient who both receives and uses gestures but no speech to communicate. Two experiments were conducted. In Experiment 1, 16 hearing adults, with no prior experience with sign language, were randomly paired to form eight dyads. The Verbs of Motion Production test (Supalla, 1982) was administered to each dyad and participants took turns as the Gesturer or Listener. The Gesturer observed a block of 10 short vignettes in which animated objects moved in varying paths and manners across space. Immediately after each vignette, the Gesturer manually expressed the event to the Listener who, in turn, offered feedback nonverbally. A total of 40 vignettes were presented so that participants alternated the role of Gesturer and Listener twice. In Experiment 2, eight hearing adults, also naive to sign language, were presented with the same set of 40 vignettes. Participants gestured the scenes to the experimenter who provided no feedback or additional input. Results indicate substantial differences in the gesture systems of individuals in the two experiments. In particular, we found that participants who engaged in mutual dialogue (1) evolved toward a more arbitrary and, therefore, less iconic system of signs, (2) had a more stable and productive lexicon of gestures, and (3) showed the beginnings of a more internally consistent and contrastive language system. The results highlight the importance of a shared communication system for the development of language, one in which individual gestures are used not only in production but are received as input by another person. Moreover, the findings have important theoretical implications for understanding how prior linguistic experience, pragmatic factors, and input conditions contribute to the development of a natural language system.